The AM/PM conversion is a measure of the amount of undesired phase deviation (PM) caused by amplitude variations (AM) inherent in the system such as a device under test with a local oscillator. In fact, the AM/PM conversion corresponds to the amount of undesired phase deviation that is caused by the amplitude variations (AM) at an input of the device under test. For instance, the unwanted phase deviations may be caused by a thermal drift of the device under test, for example the local oscillator.
Under ideal conditions, no interaction between the phase response and the power level of the input signal would occur. However, a certain AM/PM conversion occurs under real conditions. Thus, the AM/PM conversion is a critical parameter since undesired phase deviation causes analog signal degradation or rather increased bit-error rates in digital communication systems. Therefore, it is important to quantity the AM/PM conversion so that the respective interaction can be compensated effectively
In the state of the art, the AM/PM conversion is typically measured by using a spectrum analyzer wherein phase results are derived from amplitude measurements performed with the spectrum analyzer. However, this technique has some drawbacks since a multi-carrier group delay approach has to be used in addition for measuring the AM/PM conversion of a device under test having an embedded local oscillator. The multi-carrier group delay approach ensures that a frequency drifting of the local oscillator can be taken into account. However, different techniques are required for measuring the AM/PM conversion of a device under test with an embedded local oscillator.
Accordingly, there is a need for a method of measuring the AM/PM conversion of a device under test in a cost-efficient and simplified manner.